Sharks

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics. They have several sets of replaceable teeth. Sharks range in size from the small dwarf lanternshark, Etmopterus perryi, a deep sea species of only 17 centimetres in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres and which feeds only on plankton, squid, and small fish through filter feeding.


Photo: theage.com.au

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. It is a fish. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks. The first sharks appeared in the oceans 350 to 400 million years ago. Most of the species we know today are as old as the Jurassic period.

The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the blood is pumped to the shark's gills via the ventral aorta artery where it branches off into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the ventricle of the heart and the cycle repeats.

Underwater vision

Light rays bend when they enter from one medium to another of different density. The amount of bending is determined by the refractive indices of the two media. If one medium has a particular curved shape, it functions as a lens. The cornea, humours, and crystalline lens of the eye together form a lens that focuses images on the retina. Our eyes are adapted for viewing in air. Water, however, has approximately the same refractive index as the cornea, effectively eliminating the cornea's focusing properties. When our eyes are in water, instead of their focusing images on the retina, they now focus them far behind the retina, resulting in an extremely blurred image from hypermetropia.

Photo: scuba.about.com

Masks and Goggles Views through a flat mask, above and below water. By wearing a flat diving mask, humans can see clearly under water. The scuba mask's flat window separates one's eyes from the surrounding water by a layer of air. Light rays entering from water into the flat parallel window change their direction minimally within the window material itself.[1] But when these rays exit the window into the air space between the flat window and the eye, the refraction is quite noticeable. The view paths refract in a manner similar to viewing fish kept in an aquarium. Linear polarizing filters decrease visibility underwater by limiting ambient light and dimming artificial light sources.



Photo: kac-divers.com

The crystalline lenses of fishes' eyes are extremely convex, almost spherical, and their refractive indices are the highest of all the animals. These properties enable proper focusing of the light rays and in turn proper image formation on the retina.

Queen Triggerfish

The Queen Triggerfish is a large species from the tropical Atlantic and Indian Oceans. Its body is large powerful, the eyes being set high up on the forehead. The Queen Trigger is a beautiful fish that can become hand tamed in time. Watch your fingers though, the front teeth are very sharp and can deliver a painful bite. The base color of the fish is a light yellow while the dorsal and anal fins are tinted in blue.


Photo: uwphoto.net

The Queen Triggerfish have a small pelvic fin, fused to one spine. Unlike the spine of a filefish, the spine of the triggerfish can be held in place by a second spine to make the fish more threatening to the predator. Their small eyes, situated on top of their large head, can be rotated independently. They have tough skin, covered with rough rhomboid-shaped scales that form a tough armour on their body.

A big, angular-shaped head extends into a snout with strong jaws and sharp teeth, adapted for crushing shells. Some of the triggerfish species can be quite aggressive during reproduction season. In particular Picasso triggerfish and titan triggerfish viciously defend their circular nests against any intruders, including scuba divers and snorkelers. Their territory extends in a cone shape from the nest to the surface, so swimming upwards puts one further into the fishes' territory. A horizontal swim away from the nest site is the most sensible course of action when confronted by an angry triggerfish.

Jellyfish

Underwater Jellyfish

Jellyfish are most recognised because of their jelly like appearance and this is where they get their name. They are also recognised for their bell-like shape and tentacles. Jellyfish are free-swimming members of the phylum Cnidaria. They have several different basic morphologies that represent several different cnidarian classes including the Scyphozoa, Staurozoa, Cubozoa and Hydrozoa. The jellyfish in these groups are also called, respectively, scyphomedusae, stauromedusae, cubomedusae, and hydromedusae; "medusa" is another word for jellyfish. Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water. Most of the information about jellyfish that follows in this article is about scyphozoan jellyfish, or scyphomedusae. These are the big, often colorful, jellyfish that are common in coastal zones worldwide.

Underwater Jellyfish

Some of the most common and important jellyfish predators are other species of jellyfish, some of which are specialists in eating jellies. Other predators of jellyfish include tuna, shark, swordfish, and at least one species of Pacific salmon, as well as sea turtles. Sea birds sometimes pick symbiotic crustaceans from the bells of jellyfish near the surface of the sea, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp.

Jellyfish are commonly displayed in aquaria in many countries. Often the tank's background is blue and the animals are illuminated by side light to produce a high contrast effect. In natural conditions, many jellies are so transparent that they are almost impossible to see.

Holding jellyfish in captivity presents other problems. For one, they are not adapted to closed spaces. They depend on currents to transport them from place to place. To compensate for this, professional exhibits feature precise water flows, typically in circular tanks to prevent specimens from becoming trapped in corners. The Monterey Bay Aquarium uses a modified version of the kreisel for this purpose.

Sea Turtle

Sea turtles are turtles that you can found in all the oceans except the Arctic Ocean. This is the one of sea animal that I like.



The East Pacific subpopulation of the green turtle was previously classified as a separate species, the black turtle, but DNA evidence indicates that it is not evolutionarily distinct from the green turtle. There are seven living species of sea turtles: flatback, green sea turtle, hawksbill, Kemp's Ridley, leatherback, loggerhead and olive ridley.

They spend almost all lives submerged but must breathe air for the oxygen needed to meet the demands of vigorous activity. With a single explosive exhalation and rapid inhalation, sea turtles can quickly replace the air in their lungs. The lungs are adapted to permit a rapid exchange of oxygen and to prevent gasses from being trapped during deep dives. The blood of sea turtles can deliver oxygen efficiently to body tissues even at the pressures encountered during diving.